Sheet transport device

ABSTRACT

The invention relates to a sheet transport device ( 1 ) comprising a first and a second opposing transport units ( 3, 5 ), each comprising at least one circulating conveyor belt ( 15 ) and one vacuum suction device ( 9, 11 ) that is in relation with the conveyor belt, said vacuum suction devices being individually actuatable so as to attract a sheet ( 19 ), located between the transport units, to the one or the other conveyor belt. Such a sheet transport device is used, for example, in a printing machine. The invention also relates to a method for transporting a sheet to this sheet transport device.

FIELD OF THE INVENTION

The invention relates to a sheet transport device with vacuum suction,said sheet transport device being suitable for selectively guiding asheet along a sheet transport path and also allowing a sheet jam bypass.Such a sheet transport device can be used, for example, for transportingsheets in a printing machine.

BACKGROUND OF THE INVENTION

From document JP 60052455 A, a paper path switching device is known, inwhich a paper sheet to be transported is guided past air-transmissivevacuum rollers that are arranged above and below a paper path. One ofthe vacuum rollers attracts the paper sheet and selectively guides saidpaper sheet to one of two branch paths that are provided on oppositesides of the paper path in this paper path switching device.

This known paper path switching device has the disadvantage that a papersheet is only guided along a line of contact with one of theair-transmissive vacuum rollers on its sheet transport path.Consequently, the sheet may form waves upstream and downstream of thisline of contact, and sheet jams may easily occur, thus leading toproduction down times.

SUMMARY OF THE INVENTION

It is the object of the present invention to make available a sheettransport device that provides secure guiding for a sheet that is to betransported.

The object of the present invention is achieved by a sheet transportdevice in accordance with Claim 1 and by a method in accordance withClaim 20.

In particular provided is a sheet transport device comprising first andsecond oppositely arranged transport units, each of said transport unitscomprising at least one circulating conveyor belt and a vacuum suctiondevice that is in relation with the conveyor belt. Said vacuum suctiondevices can be individually actuated so as to attract a sheet, locatedbetween the transport units, to the one or the other conveyor belt. Thisoffers the advantage that the sheet is guided in a stable and steadymanner and that the probability of a sheet jam is decreased.

Advantageously, the transport units are at a distance from each othersuch that one transport unit can transport a sheet independently of therespectively other transport unit. Thus a second sheet can betransported past a first sheet if the first sheet is stuck ortransported more slowly than the second sheet.

Preferably, the vacuum suction devices comprise a mechanism for applyingdifferent vacuum pressures to different regions of the conveyor belt. Asa result of this, an inadvertent suction of a sheet from the oppositetransport unit is avoided.

This mechanism is preferably suitable for sequentially applying a vacuumto adjacent regions of the conveyor belt and has the advantage that theregions to which a vacuum has been applied can be controlledsynchronously with respect to the conveying motion of the sheet.Consequently, the conveyor belt can aspirate the sheet only in theregion that is covered by the sheet.

Advantageously, the vacuum suction device of the transport unitscomprises a suction chamber that communicates with a variablycontrollable vacuum source, which makes the variable adjustment of thevacuum possible. In order to make possible a position-dependentactivation of the transport unit with a vacuum, a suction chamber havingseveral partitions may be provided, said partitions being selectivelyconnectable to the vacuum source. The vacuum suction device may alsocomprise a suction chamber communicating with a vacuum source, saidsuction chamber having at least one suction opening that can be variablyopened and closed. By way of this, the vacuum can be controlled in asimple manner. The suction opening that is to be variably opened andclosed may comprise a variable bypass opening in order to optionallydecrease the vacuum locally. For the variable opening and closing of theat least one suction opening, preferably a flap, a valve and/or acirculating, partially air-transmissive, control web is provided, as aresult of which an adjustment of the vacuum can be achieved with simplestandard components in various regions of the transport units.

The vacuum suction devices of the opposing transport units preferablycomprise connection and disconnection features that are logically lockedwith respect to each other in such a manner that in opposite regions avacuum is applied to only one of the transport units while no vacuum oronly a minimal vacuum is applied to the other transport unit in thecorresponding region. This means, for example, that—in a region of aconveyor belt of one transport unit that is covered by the sheet—a highvacuum is applied, whereas, at the same time—in the correspondingopposite region of the other conveyor belt that is not covered by asheet—a slight vacuum is applied. This offers the advantage that a sheetconveyed by the one transport unit is not drawn over to the oppositeother transport unit.

Preferably, the sheet transport device comprises two sheet transportoutlets that are aligned with respectively one of the transport units,so that a sheet conveyed by one transport unit is guided in the sheettransport outlet aligned therewith. Thus the sheet is guided securelyand with a steady conveying movement to the sheet transport outlet.

Each of the sheet transport outlets communicates with a separate sheettransport path, said paths continuing either separately or leading backtogether into a joint sheet transport path. This results in theadvantage that the sheet transport device can be operated as a sheetswitch and also as a bypass device.

Advantageously, the sheet transport device comprises at least one sheetfeeding unit that is arranged so as to convey a sheet between the firstand the second transport units resulting in that the sheet can be easilydrawn toward one of the transport units.

In one embodiment, at least two transport units are sequentiallyprovided in transport direction of the sheets. As a result of this, itis possible to bridge longer transport paths.

In the sheet transport device, preferably at least one of the transportunits can be moved at least partially away from the opposite transportunit for removing a sheet jam or for performing maintenance work. Themovement of the transport unit may either be a pivoting movement or alinear movement.

Preferably, sheet sensors are provided for sensing a sheet position, andan activation of the vacuum suction device may take place as a functionof the output parameters of the sheet sensors. As a result of this, aselective suction of the sheets may be achieved. In particular, it ispossible to detect and, if necessary, bypass a sheet jam.

Furthermore, the object is achieved by a method for transporting a sheetfrom a sheet conveyor unit to one of two sheet transport outlets of asheet transport device comprising first and second transport units withvacuum suction devices, said devices being arranged opposite each other.With this method, depending on the sheet transport outlet to which thesheet is to be conveyed, the vacuum suction device of the first orsecond transport unit is activated in order to draw the sheet by suctionto a circulating conveyor belt of the transport units and to convey saidsheet to a sheet transport outlet. This leads to the sheet being guidedin a steady manner and the probability of a sheet jam being reduced.

The method may comprise the feeding of a first sheet between the twotransport units comprising a vacuum suction device and comprise thedetection of a sheet jam in the region of a first one of the transportunits. Subsequently, a second sheet is inserted between the twotransport units, and the second sheet is drawn by suction to the secondtransport unit and conveyed with the second transport unit. As a resultof this, a sheet may be guided past a stuck second sheet, and operationmay be upheld.

The step of detecting a sheet jam advantageously comprises themeasurement of the position of a sheet and/or the measurement of thespeed of a sheet. As a result of this, it is possible to quickly detecta sheet jam, and suction via the conveyor belt can be influenced.

Preferably, the method comprises the step of an at least partialdownward adjustment of the vacuum suction device of the transport unitthat does not transport the sheet. The step of downward adjustmentcomprises, in particular, a reduction of the vacuum level. As a resultof this, it is avoided that a sheet transported by the other transportunit is aspirated, which would cause problems.

Advantageously, the step of downward adjustment comprises a reduction ofthe vacuum level in partial regions of the vacuum suction device. Byindividually decreasing the vacuum, it is possible to generate a vacuumeffect over the length of the transport device, said effect being highwherever the sheet is seated on the transport device and being lowwherever the sheet is not seated on the transport device. It is thuspossible to achieve a concentration of the vacuum in the region where aconveyed sheet is currently located.

Advantageously, the vacuum suction device comprises a suction chamberwith several partitions, and the step of downward adjustment providesfor a reduction of the vacuum at least in one of these partitions. Thepartitions offer the simple option of defining regions of the suctionchamber and, thus, also of the transport unit, with different vacuumlevels.

In one embodiment, the suction chamber has at least one variable suctionopening, and the step of downward adjustment provides for the variableopening of this suction opening. This enables the operation of severalsuction chambers with one common vacuum source, while the vacuum in theindividual chambers can be rapidly adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, as well as additional details and advantages thereof,will be explained hereinafter with reference to preferred exemplaryembodiments and to the figures. They show in

FIG. 1 a schematic side view of a sheet transport device in accordancewith one exemplary embodiment of the present invention in a first modeof operation;

FIG. 2 a schematic side view of the sheet transport device shown in FIG.1, in a second mode of operation;

FIG. 3 a schematic side view of the sheet transport device shown in FIG.2, in a third mode of operation in which a sheet jam occurs;

FIG. 4 a schematic side view of the sheet transport device shown in FIG.1, said view illustrating a transport of sheets that is sustained duringa sheet jam;

FIG. 5 a a schematic side view of a sheet transport device in accordancewith one exemplary embodiment of the present invention, said sheettransport device comprising a pivot opening mechanism for access to asheet transport path;

FIG. 5 b a schematic side view of a sheet transport device in accordancewith another exemplary embodiment of the present invention, said sheettransport device comprising a different pivot opening mechanism foraccess to a sheet transport path;

FIG. 6 a view similar to that of FIGS. 5 a and 5 b, however with analternative opening mechanism;

FIG. 7 a a schematic side view of a transport unit of a sheet transportdevice in accordance with one exemplary embodiment of the presentinvention;

FIG. 7 b a schematic side view of a transport unit of a sheet transportdevice in accordance with another exemplary embodiment of the presentinvention; and

FIG. 7 c a schematic side view of a transport unit of a sheet transportdevice in accordance with yet another exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

It should be noted that statements such as right, left, top, bottom andsimilar statements relate to the figures and are not to be understood tohave a restrictive meaning. Furthermore, expressions such as, forexample, leading/trailing sheet or similar expressions relate to atransport direction or conveying direction of a sheet. This conveyingdirection extends from the bottom to the top in the figures.

A sheet transport device 1 in accordance with the present inventioncomprises a first transport unit 3 and a second transport unit 5, saidtransport units being arranged opposite each other on two sides of asheet transport path 7, and facing each other. The first transport unit3 comprises a first vacuum suction device 9, and the second transportunit 5 comprises a second vacuum suction device 11.

Each of the two transport units 3, 5 comprises two transport rollers 13,as well as a circulating, air-transmissive conveyor belt 15, to which avacuum can be applied by the respective vacuum suction device 9 or 11.At least one of the transport rollers 13 of a transport unit 3, 5 isdriven. Furthermore, it should be noted that a transport unit 3, 5 maybe provided with more than two transport rollers 13. Such a transportunit 3, 5 is also referred to as a suction belt mechanism.

Furthermore, the sheet transport device 1 comprises a sheet transportinlet 17 that is located adjacent to the transport units 3, 5 and fromwhich the sheets 19 are transported to the sheet transport device 1. Thesheets 19 are supplied, for example, by a (not illustrated) sheetfeeding unit, to the sheet transport inlet 17. The sheet transport inlet17 comprises two sheet transport inlet rollers 21, between which a sheet19 is guided and by which the sheet 19 is transported in the directionof the sheet transport path 7. The sheet transport inlet 17 is arrangedin such a manner that a conveyed sheet 19 arrives between the transportunits 3, 5. The vacuum suction devices 9, 11 can be selectivelyactivated, and the sheet 19 can be drawn by suction through theactivated vacuum suction device 9 or 11 toward the respectivecorresponding transport unit 3 or 5.

The figures show that the sheet 19 is fed in the center between thetransport units 3, 5. Alternatively, the sheet 19, however, may also befed outside the center between the transport units 3, 5, if it isensured that said sheet can be aspirated by an activated vacuum suctiondevice 9 or 11 and drawn by suction to the corresponding transport unit3 or 5.

It should be noted that, in another exemplary embodiment of the sheettransport device 1, it is also possible to provide several sheettransport inlets 17, these supplying different types of sheets, forexample. This plurality of sheet transport inlets 17 is arranged in sucha manner that a conveyed sheet 19 enters between the transport units 3,5 and into the region of action of the vacuum suction devices 9, 11.Depending on which of the vacuum suction devices 9, 11 is selectivelyactivated, the sheet 19 is then drawn by suction to the correspondingtransport unit 3 or 5.

Furthermore, the sheet transport device 1 comprises a first sheettransport outlet 23 and a second sheet transport outlet 25. The firstsheet transport outlet 23 is aligned with the first transport unit 3 andcomprises two opposing sheet transport output rollers 27. The secondsheet transport outlet 25 is aligned with the second transport unit 5and comprises two opposing sheet transport output rollers 29. The sheettransport outlets 23, 25 are aligned with their respective transportunits 3, 5 in such a manner that a sheet 19 conveyed by a transport unit3, 5 is guided into the associate sheet transport outlet 23, 25 and canthus be grasped by the opposing sheet transport output rollers 27 or 29.

In accordance with one exemplary embodiment, the sheet transport outlets23, 25 lead to different, continuing sheet paths 7 a, 7 b. This may bethe case, for example, in a sorting unit or when a sheet switch is used.

Alternatively, the sheet transport outlets 23, 25 may also lead to sheetpaths 7 a, 7 b that meet after a section of the path in order to againbe united to form a single sheet path 7. This may be the case when asheet 19 is to be guided around a sheet jam in the region of one of thesheet transport outlets 23, 25. Another example would be if a part ofthe sheets 19 is to be selectively guided to a processing device in oneof the sheet paths 7 a, 7 b in order to be guided back on the same sheetpath 7 for further processing.

Although the transport units 3, 5 in the figures are shown in a mannerextending parallel to each other, an alternative embodiment is beingtaken into consideration, in which the transport units 3, 5 are atcloser distances with respect to each other in the vicinity of the sheettransport inlet 17 than in the vicinity of the sheet transport outlets23, 25. Consequently, the result is a V-shaped configuration of thetransport units 3, 5, where the sheet transport outlets 23, 25 are at agreater distance from each other than is shown in FIGS. 1 through 4.

Furthermore, it should be noted that several transport units 3, 5 may besequentially provided on one side or on both sides of the sheettransport path 7 in the direction of the sheet transport path 7. This,for example, depends on the length of the sheet transport path, the typeof vacuum suction device and on whether one of the transport units 3, 5can be pivoted or shifted.

FIGS. 5 a, 5 b and 6 show exemplary embodiments, wherein at least one ofthe transport units 3, 5 can be moved away from the sheet transport path7. FIG. 5 a shows an exemplary embodiment, wherein the first transportunit 3 is pivoted away from the sheet transport path 7 and from thetransport unit 5 in the region of the upper transport roller 13. Thus itis easier for an operator to perform maintenance work such as, forexample, eliminate a sheet jam. FIG. 5 b shows an exemplary embodiment,wherein two transport units 3 a, 3 b are sequentially arranged on theright side. Consequently, this provides another alternative for accessto the sheet transport path 7 and to sheets 19 that have become stuck.

FIG. 6 shows an alternative embodiment, wherein the first transport unit3 can be linearly moved away from the sheet transport path 7. Thisprovides another alternative for access to the sheets 19 that havebecome stuck in the sheet transport path 7. It is taken intoconsideration that the movement of the transport units 3, 5 ismotor-driven. Furthermore, it is taken into consideration that, in thecase of a sheet jam, the transport units 3, 5 are automaticallytriggered to pivot away and that a return movement in one of thetransport units 3, 5 will then occur automatically in order to removethe jammed sheet.

FIGS. 7 a-c show side views of various exemplary embodiments of thevacuum suction devices 9, 11. The vacuum suction devices 9, 11 of thetransport units 3, 5 comprise a suction chamber 31 that communicateswith a vacuum source 33, thus causing a vacuum to prevail in the suctionchamber 31. This vacuum is sufficiently strong in order to draw a sheet19 toward one of the transport units 3, 5. The vacuum in the suctionchamber 31 of each transport unit 3, 5 can be adjusted independently ofthe suction chamber 31 of the other transport unit 3, 5. Viewed in thedirection of length, the suction chamber 31 extends essentially acrossthe entire space between the transport rollers 13 and, viewed in thedirection of width, across the width of the conveyor belt 15. Thesuction chamber 31 has a plurality of sheet suction openings 34 thatface toward the air-transmissive conveyor belt 15 and are essentiallyuniformly distributed. When the vacuum source 33 is switched on, the airis sucked through the air-transmissive conveyor belt 15 and into thesheet suction openings 34.

In accordance with one exemplary embodiment, the vacuum source 33 can bevariably adjusted, for example by means of a blower featuring differentspeeds and/or by a flap or valve control. In this kind of arrangement,the total vacuum in the suction chamber 31 changes and—overall—more orless air is sucked through the sheet suction openings 34.

In another exemplary embodiment, each of the transport units 3, 5comprises at least one control element (not shown in the figures) thatis air-transmissive, at least in partial areas. The control element canbe moved into a position adjacent to the sheet suction openings 34 andhas dimensions such that it is able to completely, or also onlypartially, cover the sheet suction openings 34. In particular, viewed intransport direction of the conveyor belt 15, the control element maysequentially cover and clear adjacent regions. Depending on itsposition, the control element may cover parts of the sheet suction holes34. As a result of the covered sheet suction holes 34, it is notpossible to draw in air by suction. Alternatively, the control elementmay also comprise partial regions that are less air-transmissive thanthe sheet suction openings 34. When such partial regions cover the sheetsuction openings 34, the air flow is only reduced and not completely cutoff as a result of this. A control web, or also adjacent flap elementsin the suction chamber 31, could be used as the control element. Whenusing the control web, for example, half of the control web may beair-tight or less air-transmissive so that, depending on the position ofthe control web, all the sheet suction openings 34 may be selectivelyfree or covered.

In a further exemplary embodiment, the vacuum suction devices 9, 11comprise suction chambers 31 having several compartments 35 with theirown sheet suction openings 34 that can be selectively connected to thevacuum source 33. Consequently, a part of the compartments 35 maydisplay a lesser vacuum than the other compartments 35.

The distribution of the vacuum in the compartments 35 can be adjusted independence of the position of a conveyed sheet 19. In particular thevacuum is switched off in those compartments 35, where there is noconveyed sheet 19, as is indicated by crossed-out arrows in FIGS. 1through 4. For example, the compartments 35 are connected with thevacuum source 33 by means of separate lines or via a manifold havingseveral valves.

In accordance with a further exemplary embodiment, the suction chamber31 comprises a variable suction opening 37. In FIG. 7 c, the variablesuction opening 37 is shown as a variable bypass opening by way ofwhich—in open state—additional air is aspirated, said air not beingdrawn by suction through the air-transmissive conveyor belt 15 andthrough the sheet suction openings 34. The vacuum in the suction chamber31 varies as a function of the degree of opening of the variable suctionopening 37.

Alternatively, the suction opening 37 is identical with the sheetsuction openings 34. This means that the sheet suction openings 34 canbe variably opened and closed, so that the vacuum suction in partialregions of the air-transmissive conveyor belt 15 may be adjusteddownward.

It should be noted that a suction chamber 31 may be connected with avariable vacuum source as well as also comprise several compartments 35,and also comprise a variable suction opening 37. For example, thevariable suction opening 37 may be elongated and extend over the entirelength of the suction chamber 31. When the variable suction opening 37is gradually opened, it sequentially opens several compartments 35toward the environment. Consequently, independently of the openingdegree of the variable suction opening 37, air is sucked into one ormore of the compartments 35 from the environment.

In the exemplary embodiment that uses the control element, said elementis aligned with the sheet 19 in such a manner that the part of thesuction opening 37 that is not covered by the sheet 19 is covered by thecontrol element. Thus air is aspirated only through the part of theconveyor belt on which the sheet 19 is located.

The strength of the vacuum and the method of application used by thevacuum suction device is adjusted as a function of the position of asheet 19 that is to be conveyed. The position and/or the speed of thesheet 19 are detected by one or more not illustrated sheet detectorsalong the sheet transport path 7. These sheet detectors may be, forexample, light barriers, touch-sensitive sensors, air pressure sensors,etc.

Hereinafter, the operation of the sheet transport device will beexplained in greater detail. In accordance with one exemplaryembodiment, the sheet transport device 1 is used to transport a sheet 19along the sheet transport path 7 and to feed said sheet to one of thetwo sheet transport outlets 23, 25.

First, the sheet 19 is conveyed, coming from the sheet transport inlet17, between the transport units 3 and 5. A vacuum is selectively appliedto one of the vacuum suction devices 9, 11, while the vacuum of theother suction device is switched off. Consequently, as is shown in FIG.1, the sheet 19 is drawn to the right toward the transport unit 3. Asillustrated by the arrows in FIG. 1, a vacuum is applied to the suctionchamber 31 of the right transport unit 3. As indicated by thecrossed-out arrows in FIG. 1, no vacuum is applied to the suctionchamber 31 of the second transport unit 5. The vacuum in the suctionchamber 31 draws the sheet 19 toward the conveyor belt 15 of the righttransport unit 3, and said conveyor belt then conveys the sheet 19 inthe direction of the first sheet transport outlet 23. There, the sheet19 is grasped by the opposing sheet transport output rollers 27 andcontinued to be conveyed.

Alternatively, as can be seen in FIG. 2, a sheet 19 transported from thesheet transport inlet 17 is pulled by vacuum toward the left transportunit 5, while the vacuum in the suction chamber 31 of the righttransport unit 3 is switched off. In this case, the left transport unit5 conveys the sheet 19 to the second sheet transport outlet 25, wheresaid sheet is grasped by the opposing transport rollers 29 and continuedto be transported.

Consequently, the sheet 19 is selectively drawn either toward the righttransport unit 3 or toward the left transport unit 5, and is thustransported either to the right sheet transport outlet 23 or to the leftsheet transport outlet 25. In this mode of operation, the sheettransport device 1 acts as a switch for a sheet 19 that is guided eitherto the sheet path 7 a or 7 b.

The vacuum suction devices 9, 11 of the opposing transport units 3, 5thus feature an on-switching and an off-switching that can be logicallylocked with respect to each other in such a manner that a vacuum isapplied to only one of the transport units 3, 5, while no vacuum or onlya minimal vacuum is applied to the other transport unit 3 or 5 in thecorresponding region. This means, for example, that—in a region of aconveyor belt that is covered with a sheet—a high vacuum is applied tothe one transport unit, while, at the same time, a minimal vacuum isapplied to the opposite region of the other conveyor belt that is notcovered by a sheet. This may be achieved, for example, by flaps in themanifolds to the vacuum source 33, said manifolds being arranged in sucha manner that they alternately connect a region of a transport unit 3, 5with the vacuum source 33, while they, at the same time, cut off theopposing region of the other transport unit 3 or 5 from the vacuumsource 33.

Another example of the method in accordance with the present inventionallows a bypassing of a sheet around a sheet jam in a transport path 7 aor 7 b. The method for bypassing a sheet 19 will be explained in detailhereinafter with reference to FIG. 3.

A first sheet 19 has been conveyed from the direction of the sheettransport inlet 17 and has been transported further by the transportunit 3 to the first sheet transport outlet 23 (sheet transport path 7a). However, a sheet jam has occurred at the first sheet transportoutlet 23 due to a problem, so that the first sheet 19 can no longer betransported further. The sheet jam and the position of the sheet 19 aredetected by means of output signals of the sheet detectors.

In one exemplary embodiment, a sheet jam in the sheet transport path 7is detected by detection of the position of a sheet 19 in the sheettransport path 7. One or more (not illustrated) detectors determine theposition of a sheet 19. If the position has not changed for a certainperiod of time, a sheet jam is detected. In accordance with a furtherexemplary embodiment, a sheet jam is detected by detecting the speed ofa sheet 19 in the sheet transport path 7. As soon as the speed dropsbelow a previously specified level or the speed of the sheet 19 deviatesgreatly from the speed of the transport unit 3 or 5 transporting saidsheet, a sheet jam is detected. Alternatively, a sheet jam is detectedwhen the speed of the leading edge of the sheet 19 deviates from thespeed of the trailing edge of the sheet 19.

Following the detection of a sheet jam, the transport unit 3 or 5, inwhose region the sheet jam has occurred, is stopped. As a result ofthis, it is avoided that the sheet jam becomes denser or that the sheet19 will undulate and obstruct the nip between the transport units 3 and5.

As indicated by the arrows pointing to the right in FIG. 3, the rearpart of the sheet 19 that is still located in the region of the suctionchamber 31 of the first transport unit 3 continues to be attracted bythe vacuum in order to prevent that said rear part will lift off thetransport unit 3 and thus cause additional problems. As shown by thecrossed-out arrows pointing to the right in FIG. 3, the vacuum in theexposed region of the transport unit 3, however, is decreased orswitched off.

Then, another sheet 19 is supplied from the direction of the sheettransport inlet 17 and is pulled—due to the vacuum—toward the left tothe second transport unit 5. Thereafter, the second transport unit 5transports the second sheet 19 in the direction of the second sheettransport outlet 25, where said sheet is grasped by the sheet transportoutput rollers 29 and conveyed further (sheet transport path 7 b).Consequently the sheet jam in the first sheet transport outlet 23 can bebypassed in that the second sheet 19 is conveyed through theunobstructed transport unit 5 past the stuck first sheet 19.

Although not specifically shown in the figures, the second sheet 19 andadditional subsequent sheets 19 can be conveyed further through thesecond transport unit 5 and the sheet transport output rollers 29 alongthe sheet transport path 7 b and then be guided back to the originalsheet transport path 7. Thus the sheet jam in the first sheet transportoutlet 23 is bypassed, and the operation of the sheet transport device 1need not be interrupted.

As is shown in FIG. 3 by left-pointing crossed-out arrows, the vacuum inthe regions of the suction chamber 31 of the left transport unit 5 isswitched off or decreased. As a result of this, it is prevented that thestuck first sheet 19 is drawn toward the second transport unit 5 andcauses another sheet jam. The vacuum in the suction chamber 31 of theleft transport unit 5 is at full level only in the region where thesecond sheet 19 is situated on the transport unit 5.

It should be pointed out that FIGS. 1 and 2 show a situation, in whichtwo sheets 19 are aspirated and transported at a short distance fromeach other by the right transport unit 3. In this case, the vacuum inthe suction chamber 31 is constant across the entire transport unit 3.In contrast, FIG. 4 shows a situation in which two sheets 19 areconveyed at a greater distance by the left transport unit 5. As is shownin FIG. 4 by the crossed-out arrows, the vacuum is switched off, or atleast decreased, in the region that is not covered by one of the sheets19.

As has already been mentioned above, the vacuum suction device of atleast the transport unit 3, 5 that does not transport the sheet 19(i.e., the transport unit 3 or 5 with the sheet jam) is partiallyadjusted downward while a sheet jam is being detected and while a sheet19 is being transported. In an exemplary embodiment, this process of adownward adjustment occurs by means of a general reduction of the vacuumlevel of one of the vacuum suction devices 9, 11. The reduction of thevacuum level is achieved, for example, by the downward adjustment of ablower or by flaps or valves in the connection between the suctionchamber 31 and the vacuum source 33.

In accordance with another exemplary embodiment, the vacuum level isreduced only in partial regions of the vacuum suction device 9, 11. Incase a suction chamber 31 comprises several compartments 35, thereduction of the vacuum level displays the reduction of the vacuum in aleast one of these compartments 35. Inasmuch as the compartments 35communicate with the vacuum source, either individually or via amanifold, the change of the vacuum is achieved by opening or closing offlaps or valves in the connecting lines.

In an embodiment wherein the suction chamber 31 comprises a variablesuction opening 37, the step of downward adjustment of the vacuumsuction device comprises the variable opening of said suction opening37. FIG. 7 c shows the variable suction opening 37 as a variable bypassopening, by way of which additional air is aspirated when in open state.Additional air that has been sucked in through the bypass opening is notsucked through the air-transmissive conveyor belt 15 and through thesheet suction openings 34. As a result of this, the vacuum used forattracting the sheet 19 is decreased. The vacuum in the suction chamber31 thus varies as a function of the opening degree of the variablesuction opening 37.

Alternatively, the suction opening 37 is identical with the sheetsuction openings 34. In this case, the step of downward adjustment ofthe vacuum suction device comprises the variable opening of the sheetsuction openings 34. Thus the vacuum suction can be adjusted downward inpartial regions of the air-transmissive conveyor belt 15, wherein thesheet suction openings 34 are closed further. In accordance with oneembodiment, the sheet suction openings 34 are opened or closed byselectively activatable valves, slides or flaps. In accordance withanother embodiment, the sheet suction openings 34 are closed orpartially closed by the above-described control web.

A suction chamber 31 may communicate with a variable vacuum source, maycomprise several compartments 35 and may also comprise a variablesuction opening 37. In such a case, a combined actuation of theseelements for adjusting the vacuum can provide great flexibility. Forexample, the variable suction opening 37 may be elongated and extendalong the entire length of the suction chamber 31. When the variablesuction opening 37 is gradually opened, said variable suction openingopens several compartments 35 in sequence toward the environment.Consequently, depending on the opening degree of the variable suctionopening 37, air from the environment is sucked into one or more of thecompartments 35.

The invention has been described with reference to preferred exemplaryembodiments, where the individual features of the described exemplaryembodiments may be freely combined and/or interchanged with each other,provided they are compatible. Numerous modifications and configurationsare possible for and obvious to the person skilled in the art without adeparture from the inventive idea.

The invention claimed is:
 1. Sheet transport device comprising: firstand second sheet transport outlets; first and second opposing transportunits aligned with the respective transport outlets and set at adistance from each other such that one transport unit can transport asheet independently of the other transport unit, and a sheet conveyed byone transport unit is guided in the sheet transport outlet alignedtherewith; wherein each transport unit includes a circulating conveyorbelt and vacuum suction device in operative arrangement with theconveyor belt; the vacuum suction devices are individually actuatable soas to attract a sheet, located between the transport units, to the oneor the other conveyor belt; each vacuum suction device includes amechanism for applying different vacuum pressures to different regionsof the conveyor belt; and at least one of the transport units is atleast partially movable away from the opposing transport unit.
 2. Thesheet transport device according to claim 1, wherein each vacuum suctiondevice is adapted to sequentially apply a vacuum to adjacent regions ofthe corresponding conveyor belt.
 3. The sheet transport device accordingto claim 1, each vacuum suction device including a suction chamber and avariably controllable vacuum source in communication therewith.
 4. Thesheet transport device according to claim 1, each vacuum suction deviceincluding a suction chamber with several partitions, the partitionsbeing selectively connectable to a vacuum source.
 5. The sheet transportdevice according to claim 1, each vacuum suction device including asuction chamber communicating with a vacuum source and having a suctionopening that can be variably opened and closed.
 6. The sheet transportdevice according to claim 5, wherein the suction opening includes avariable bypass opening operative to decrease the vacuum.
 7. The sheettransport device according to claim 5, further including a flap, avalve, or a circulating, partially air-transmissive, control web adaptedto open and close the suction opening.
 8. The sheet transport deviceaccording to claim 1, each vacuum suction device including connectionand disconnection features so that that a vacuum is applied to only oneof the transport units in opposing regions, while no vacuum or only aminimal vacuum is applied to the other transport unit in thecorresponding region.
 9. The sheet transport device according to claim1, wherein each sheet transport outlet communicates with a separatesheet transport path.
 10. The sheet transport device according to claim1, further including a sheet feeding unit arranged to convey a sheetbetween the first and the second transport units.
 11. The sheettransport device according to claim 1, further including an additionaltransport unit provided after the first or the second transport unit ina transport direction of the sheets.
 12. The sheet transport deviceaccording to claim 1, wherein the at least one of the transport units isadapted to pivot.
 13. The sheet transport device according to claim 1,wherein the at least one of the transport units is adapted to movelinearly.
 14. The sheet transport device according to claim 1, furtherincluding sheet sensors for sensing a sheet position, wherein at leastone if the vacuum suction devices is activated in response to outputparameters of the sheet sensors.
 15. A method for transporting a sheetto one of two sheet transport outlets, comprising: providing a sheetconveyor unit and a sheet transport device, the sheet transport deviceincluding first and second transport units with circulating conveyorbelts and vacuum suction devices, the devices being arranged oppositeeach other and spaced apart from each other; feeding a first sheetbetween the two transport units; depending on the sheet transport outletto which a first sheet is to be conveyed, activating the vacuum suctiondevice of the first or second transport unit to draw the first sheet bysuction to the conveyor belt of the corresponding transport unit;conveying the first sheet to the corresponding sheet transport outletusing the corresponding conveyor belt; detecting a sheet jam in theregion of the corresponding transport unit, whereby the correspondingtransport unit is an obstructed transport unit and the other of thefirst and second transport units is an unobstructed transport unit;feeding a second sheet between the two transport units; and conveyingthe second sheet using the unobstructed transport unit.
 16. The methodaccording to claim 15, wherein the detecting step includes measuring theposition of a sheet.
 17. The method according to claim 15, wherein thedetecting step includes measuring the speed of a sheet.
 18. The methodaccording to claim 15, wherein the detecting step includes stopping thetransport unit having the sheet jam.
 19. The method according to claim15, further including adjusting downward, at least partially, the vacuumsuction device of the transport unit that does not transport the sheet.20. The method according to claim 19, wherein the adjusting stepincludes reducing the vacuum level.
 21. The method according to claim19, wherein the adjusting step includes reducing the vacuum level inpartial regions of the vacuum suction device.
 22. The method accordingto claim 15, wherein the vacuum suction device includes a suctionchamber with several partitions, and adjusting step includes reducingthe vacuum at least in one of the partitions.
 23. The method accordingto claim 22, wherein the vacuum suction device has a suction chamberwith a variable suction opening, and the adjusting step includesvariably opening the suction opening.